Need a new battery. Single or double?

My NH 575E has always started draggy. It generally starts okay in the summer but has always had to be jumped off in the winter. Winters aren't that bad here. Even in summer it spins slow but usually starts. I have replaced the alternator and starter in the recent past and as far as I can tell neither those or the cables are my problem. When I first got it I replaced the cable ends with tinned brass solder on ends.

My tractor came with a single battery vs the dual setup. I think I have a 850...

Need a new battery. Single or double?